One way of controlling the temperature in individual comfort zones of multi-zone temperature conditioning systems, is to control the flow rate of heated (or cooled) air supplied to the zone. This is generally accomplished by controlling the position of a supply air valve in response to an air flow sensor and thermostat.
In a typical VAV system, computer controlled converters apply varying pneumatic pressure to VAV valve actuators causing the valve(s) to modulate air flow through distribution ducts, thereby controlling the temperature and degree of ventilation in the individual zones. In such a system there may be as many as 1000 VAV valves and a corresponding number of converters in the vast array of distribution ducts. Such a large pneumatic system requires the use of large air compressors to maintain the necessary system pressure. Such pneumatic systems are also very costly to install and maintain, and consume a great deal of power.
In an alternative approach, pneumatic thermostats are used to control the temperature in the comfort zones. A thermostat in each zone is typically supplied with air at a constant pressure from a remotely located compressor and pressure regulator, or from the system itself (i.e. system powered).
A typical pneumatic thermostat is disclosed in U.S Pat. No. 4,017,025. The noted patent utilizes two separate supplies of air pressure from a distribution duct to effect temperature responsive control of flow in a variable air volume system. A bellows is adapted to bias a bleed valve to close in response to temperature, and the resulting pressure increase in the control line associated with the bleed valve biases a second bleed valve to close, thereby effecting control of flow in the duct. The control is also operative to maintain a constant volume flow in the duct with respect to changes in duct pressure, by means of flow sensors biasing the bleed valve associated with the temperature responsive bellows.
The pneumatic thermostat disclosed in U.S. Pat. No. 4,283,008 includes means to regulate air flow through a distribution duct. The flow regulator is disclosed as a bellows diaphragm which is adapted to move a valve member in a linear direction relative to the duct. As the valve member is displaced, air flow through apertures in the apparatus housing is varied, thereby affecting flow rate. A drawback of most pneumatic thermostat systems is that they similarly require the use of large air compressors. Further, each system requires the installation of relatively long runs of pneumatic tubing at considerable expense.
The need therefore exists for VAV valves that are system powered and do not require auxiliary supplies of compressed air or long runs of pneumatic tubing for their actuation.